This invention relates to a fin tab device for making compensative adjustment for straightaway or straight-line movement of vessels, and more specifically to a trim tab device of this character which functions also as an anode for marine vessels.
In general, in an outboard engine, an inboard engine unit with outboard drive unit, etc. of, for example, 30 horsepower or more, a fin tab is provided in the slipstream or race produced by a propeller of a vessel (see Japanese Patent Appln. Laid-Open Publication No. 59-70292. The fin tab is acted upon by the flow of water produced by the rotation of the propeller to prevent the production of a steering biasing force on the vessel when the helmsman intends to propel the vessel in a straight-line course, thereby facilitating the steering of the vessel. Furthermore, the fin tab is made of, for example, zinc (Zn) alloy and serves as an anode. That is, galvanic or electrolytic corrosion is caused to concentrate on the fin tab to thereby protect other components of the vessel and the outboard engine from being corroded.
The rotation of a propeller produces cavitation behind the outer tips of the propeller blades. The bubbles of the cavitation strike the fin tab and collpase, imparting impact forces to the fin tab. But the fin tab, which functions as an anode as well and is made of a material of weak mechanical strength such as zinc (Zn) alloy or the like, is subjected not only to the erosive wear due to corrosion but also to the forces due to the cavitation, and the lower portion of the fin tab is severly attrited. As a consequence, the fin tab ordinarily is replaced with a new one every year or every two years. That is, the service life of the fin tab is short.
In view of the above described problems of the prior art fin tab the object of this invention is to provide a fin tab device for marine vessels which has a long service life.